What was the last plague?

Answer: Death of the firstborn

The LORD announced one more plague; at midnight He struck down every firstborn in Egypt, from Pharaoh's son to the prisoner's.

“and every firstborn son in the land of Egypt will die, from the firstborn of Pharaoh who sits on his throne, to the firstborn of the servant girl behind the hand mill, as well as the firstborn of all the cattle.”— Exodus 11:5 (BSB)